1. Anatomy of the Human Liver
The primary structural and functional unit of the mature liver is the acinus, which in cross section is organized like a wheel around two distinct vascular beds: 3-7 sets of portal triads (each with a portal venule, hepatic arteriole, and a bile duct) for the periphery, and with the central vein at the hub. The liver cells are organized as cell plates lined on both sides by fenestrated endothelia, defining a series of sinusoids that are contiguous with the portal and central vasculature. Recent data have indicated that the Canals of Hering, small ducts located around each of the portal triads, produce tiny ductules that extend and splice into the liver plates throughout zone 1 forming a pattern similar to that of a bottle brush (Theise, N. 1999 Hepatology. 30:1425-1433).
A narrow space, the Space of Disse, separates the endothelia from hepatocytes all along the sinusoid. As a result of this organization, hepatocytes have two basal domains, each of which faces a sinusoid, and an apical domain which is defined by the region of contact between adjacent hepatocytes. The basal domains contact the blood, and are involved in the absorption and secretion of plasma components, while the apical domains form bile canaliculi, specialized in the secretion of bile salts, and are associated through an interconnecting network with bile ducts. Blood flows from the portal venules and hepatic arterioles through the sinusoids to the terminal hepatic venules and the central vein.
Based on this microcirculatory pattern, the acinus is divided into three zones: zone 1, the periportal region; zone 2, the midacinar region, and zone 3, the pericentral region. Proliferative potential, morphological criteria, ploidy, and most liver-specific genes are correlated with zonal location (Gebhardt, R., et al. 1988. FEBS Lett. 241:89-93; Gumucio, J. J. 1989, Vol. 19. Springer International, Madrid; Traber, P. et al. 1988. Gastroenterology. 95:1130-43). Gradients in the concentration of blood components, including oxygen, across the acinus, and following the direction of blood flow from the portal triads to the central vein, are responsible for some of this zonation, for example the reciprocal compartmentation of glycolysis and gluconeogenesis. However, the periportal zonation of the gap junction protein connexin 26 and the pericentral zonation of glutamine synthetase, to name only two, are insensitive to such gradients, are more representative of most tissue-specific genes and appear to be determined by factors intrinsic to the cells or to variables other than blood flow in the microenvironment.
In addition to hepatocytes, bile duct epithelial cells (cholangiocytes), and endothelial cells, the region between the portal and central tracts contains other cell types, such as Ito cells and Kupffer cells. These play prominent roles in pathogenic conditions of the liver, especially in inflammation and fibrosis, but their direct contribution to the main homeostatic functions of the normal organ are apparently small.
2. Development of the Human Liver
The liver develops as a result of the convergence of a diverticulum formed from the caudal foregut and the septum transversum, part of the splanchnic mesenchyme. The formation of the hepatic cells begins after the endodermal epithelium interacts with the cardiogenic mesoderm, probably via fibroblast growth factors. The specified hepatic cells then proliferate and penetrate into the mesenchyme of the septum transversum with a cord like fashion, forming the liver anlage. The direct epithelial-mesenchymal interaction is critical in these early developmental stages of the liver and dictates which cells will become hepatocytes or cholangiocytes, and the fenestrated endothelia, respectively. Mutations in the mesenchyme-specific genes hlx and jumonji block liver development, illustrating the importance of contributions from this tissue. Early in its development, the liver consists of clusters of proximal hepatic stem cells bounded by a continuous endothelium lacking a basement membrane and abundant hemopoietic cells. As the endothelium is transformed to become a discontinuous, fenestrated endothelium, the vasculature, especially the portal vasculature, becomes more developed with the production of basement membranes. The portal interstitium may provide the trigger for the development of bile ducts, and as it surrounds the portal venules, hepatic arterioles, and bile ducts, portal triads are formed. Proximal hepatic stem cells rapidly proliferate and parenchymal plates are formed, probably in response to changes in the amount and distribution of such tissue-organizing molecules as C-CAM 105, Agp110, E-cadherin, and connexins, coincident with the relocation of most, but not all, of the hemopoietic cells to the bone marrow. Recent studies suggest that some hemopoietic progenitors persist in the adult quiescent rodent liver, and hemopoietic stem cells have been isolated from both adult human and murine liver (Crosbie, O. M. et al. 1999. Hepatology. 29:1193-8).
The rat liver forms in embryonic life at about day 10, referred to as “embryonic day 10” or E10, with the invagination of the cardiac mesenchyme by endoderm located in the midgut region of the embryo (Zaret, K. 1998. Current Opinion in Genetics & Development. 8:526-31). Earliest recognition of liver cells in the embryos has been achieved by using in situ hybridization studies for mRNA encoding alpha-fetoprotein (AFP) ((Zaret, K. 1998. Current Opinion in Genetics & Development. 8:526-31; Zaret, K. 1999 Developmental Biology (Orlando). 209:1-10). AFP-expressing cells are observed in the midgut region of the embryo near the mesenchyme that produces the heart on day 9-10 in all rat and mouse livers assayed. The liver becomes macroscopically visible by E12 and is about 1 mm in diameter by E13.
In parallel, hemopoiesis occurs with the first identifiable hemopoietic cells appearing by E15-E16 (in rodents) and by the 3rd to 4th month (in humans) and with the peak of erythropoiesis (formation of erythroid cells or red blood cells) occurring by E18 (in rodents) and by the 5th-6th month (in humans). At the peak of erythropoiesis, the numbers of these red blood cells dominate the liver and account for more than 70% of the numbers of cells in the liver. The end of the gestational period is on day 21 in rodents and 9 months in humans. Within hours of birth, the numbers of hemopoietic cells decline dramatically such that by 2 days postnatal life (rodent) and within a week or two (human), the vast majority of the hemopoietic cells have disappeared having migrated to the bone marrow. No one knows the cause for the migration of the hemopoietic cells. There are however two dominant speculations.
First, the hemopoietic progenitors prefer relatively anaerobic conditions and most of them migrate to the bone marrow (which is relatively anaerobic) with the elevated oxygen levels in the liver with the activation of the lungs. In addition, there have speculations that the loss of the pregnancy hormones may also be a factor in the migration. Postnatally, the loss of the hemopoietic progenitors in the liver is correlated with a dramatic reduction in the numbers of hepatic progenitors and a parallel increase in the numbers and maturity of the hepatocytes and biliary cells. Full maturity of the liver is completed by 2-3 weeks postnatal life (in rodents) and within a few months (humans). By then the remaining hepatic progenitor cells are localized to the regions of Canals of Hering, with the dominant numbers of them present the portal triads in the periphery of each liver acinus (Thiese et al, Crawford et al.).
Thereafter, the classic architecture of the liver acinus is established with each acinus being defined peripherally by six sets of portal triads, each one having a bile duct, an hepatic artery and an hepatic vein, and in the center a central vein that connects to the vena cava. Plates of liver cells, like spokes in a wheel, extend from the periphery to the center. By convention, the plates are divided into three zones: Zone 1 is near the portal triads; zone 2 is midacinar; and zone 3 is near the central veins. The only diploid cells of the liver are in zone 1; tetraploid cells are in zone 2; and tetraploid, octaploid and multinucleated cells are in zone 3. The pattern is highly suggestive of a maturational lineage that ends in an apoptotic process (Sigal, S. H., S. et al. 1995. Differentiation. 59:35-42).
3. Liver Disease
Each year in the United States, there are about 250,000 people hospitalized for liver failure. Liver transplants are curative for some forms of liver failure, and approximately 4100 transplants are performed a year in United States. One of the limiting factors in liver transplantation is the availability of donor livers especially given the constraint that donor livers for organ transplantation must originate from patients having undergone brain death but not heart arrest. Livers from cadaveric donors have not been successful, although recent efforts to use such donors have supported the possibility of using them if the liver is obtained within an hour of death.
Cell transplantation into the liver is an attractive alternative therapy for most liver diseases. The surgical procedures for cell transplantation are minor relative to those needed for whole organ transplantation and, therefore, can be used for patients with various surgical risks such as age or infirmity. The use of human liver cells is superior to liver cells derived from other mammals because the potential pathogens, if any, are of human origin and could be better tolerated by patients and could be easily screened before use.
Attempts to perform liver cell transplantation have made use of unfractionated mature liver cells and have shown some measure of efficacy (Fox, I. J. et al. 1998. New England Journal of Medicine. 338:1422-1426). However, the successes require injection of large numbers of cells (2×1010), since the cells do not grow in vivo. Furthermore, the introduction of substantial numbers of large mature liver cells (average cell diameter 30-50 μm) is complicated by their tendency to form large aggregates upon injection, resulting in potentially fatal emboli. Moreover, these cells elicit a marked immunological rejection response forcing patients to be maintained on immunosuppressive drugs for the remainder of their lives. Finally, mature liver cells have not been successfully cryopreserved and complicated logistics are required to coordinate the availability of suitable liver tissue, the preparation of cell suspensions and the immediate delivery of the cells for clinical therapies.
4. Totipotent Stem Cells
Stem cells are an alternative cell-based therapy for liver disease. Totipotent stem cells are primitive cells that can self-replicate, are pluripotent, i.e. produce daughter cells with more than one fate, that can expand extensively and that can give rise to determined stem cells that can reconstitute a tissue or tissues. Most of the literature on stem cells derives either from the literature on embryos or that on hemopoietic, epidermal, or intestinal tissues.
More recently, the definitions have been modified to recognize particular classes of stem cells. Those with the potential to participate in the development of all cell types including germ cells are referred to as totipotent stem cells and include the zygote and normal embryonic cells up to the 8 cell stage (the morula). Embryonic stem cells, also called “ES” cells, consist of permanent cell populations derived from totipotent, normal cells in blastocysts, that were first reported in the early 1980s. ES cell lines can be cultured in vitro with maintenance of totipotency. When ES cells are injected back into normal blastocysts, they are able to resume embryonic development and participate in the formation of a normal, but chimeric, mouse. Although ES cell lines have been established from many species (mouse, rat, pig, etc.), only the mouse system has been used routinely to generate animals with novel phenotypes (knockouts, transgenics) by merging modified ES cells from culture to blastocysts and then implanting the blastocysts into pseudopregnant hosts. Embryonic germ (EG) cell lines, which show many of the characteristics of ES cells, can be isolated directly in vitro from the primordial germ cell population. As with ES cells, the EG cells contributed to chimeras, including the germ line, when injected into blastocysts.
Recent, highly publicized experiments have reported that human ES cell cultures can be established from human embryos. It has been suggested that these human ES cells may be injected into tissues in the hope that they will be able to reconstitute damaged organs and tissues. However, ES and EG cells are tumorigenic if introduced into immunocompromised hosts in any site other than in utero, forming teratocarcinomas. Therefore, the plan to inoculate human ES cells into patients is unrealistic and with the grave possibility of creating tumors in the patients. To overcome this impasse, some groups are pursuing the plan of differentiating the ES cells under defined microenvironmental conditions to become determined stem cells that can then be safely inoculated into patients. For example, there is some measure of success in generating hemopoietic progenitors. However, the concern remains that residual ES cells in the culture could pose the risk of tumorigenesis, if the cultures are inoculated into a patient. In summary, until research in developmental biology reveals the myriad controls dictating the fates of cells during embryogenesis, the ES cells will remain as an experimental tool with little hope for clinical programs in cell or gene therapies. The only realistic option for clinical programs in cell and gene therapies is to use determined stem cells in which the genetic potential is restricted to a limited number of cell types.
5. Determined Stem Cells
Determined stem cells are pluripotent cells that have restricted their genetic potential to that for a limited number of cell types and have extensive growth potential. Increasing evidence such as that from the telomerase field suggest that determined stem cells do not, strictly speaking, self-replicate, that is their progeny can have less growth potential than the parent. Determined stem cells give rise to committed progenitors, daughter cells that lose pluripotency by restricting their genetic potential to a single fate, e.g. hepatocytes, whose committed progenitors are referred to as committed hepatocytic progenitors. In the hepatic lineage there are committed hepatocytic progenitors (giving rise to hepatocytes) and committed biliary progenitors (giving rise to bile ducts).
The transitions from the stem cell to the adult cells occur in a step-wise process yielding a maturational lineage in which cell size, morphology, growth potential and gene expression is tied to the lineage. The metaphor of aging is useful in defining the process. The “young” cells have early gene expression and the greatest growth potential; the cells late in the lineage have “late” gene expression and usually are limited in their growth or do not grow at all. The late cells can be considered “old” or in biological terms, apoptotic, and ultimately are sloughed off. The maturational lineage process results in a natural turnover for the tissue and allows for regeneration after injuries. Tissues differ in the kinetics of the maturational process. The maturational lineage of the gut is quite rapid with a complete cycle occurring in less than a week; that of the liver is slow occurring, and in the rat liver is about a year.
There is a strong clinical and commercial interest in isolating and identifying immature progenitor cells from liver because of the impact that such cell population may have in treating liver diseases. The use of hepatic progenitors in cell and gene therapies can overcome many of the shortcomings associated with use of mature liver cells described above. The cells are small (7-15 μm), therefore minimizing the formation of large emboli. Also, the cells have extensive growth potential meaning that fewer cells are needed for reconstitution of liver tissue in a patient. Finally, the progenitors have minimal antigenic markers that might elicit immunological rejection providing hope that little or no immunosuppressive drugs might be needed.
6. Isolation of Liver Progenitors
Isolation of liver progenitors from liver is known to be an extremely challenging task due to the shortage of markers that positively select for liver cells. The only available antibodies for candidates of hepatic progenitors are those monoclonal antibodies that are prepared against subpopulations of hepatic progenitors, called oval cells if isolated from hosts exposed to oncogenic insults. These antibodies however cross-react with antigens present in hemopoietic cells.
The term oval cells is derived from a myriad of studies in the fields of carcinogenesis and oncogenesis. Animals exposed to carcinogens or other oncogenic insults experience a dramatic loss of mature liver cells (killed by the various insults) and, secondarily, expansion of small cells (7-15 μm in diameter) with oval-shaped nuclei and bearing markers that comprised both hepatic and hemopoietic antigens (Grisham and Thorgeirrson, 1998). The studies on oval cells led to the hypotheses that they are hepatic progenitors that are triggered to expand under the conditions of the oncogenic insults and that with the proper conditions can go on to be tumor cells. The phenotype of the oval cells varies in subtle and not subtle ways depending on the oncogenic insult(s). Moreover, they are known to be readily established in culture without special feeders or medium conditions. (J. Grisham and S. Thorgeirrson, 1998, Hepatic Stem Cells, In: Stem Cells, C Potten, editor, Academic Press, NY). Based on these findings and on studies characterizing some of the cell lines derived from the oncogenic treatments, it was realized that liver tumors are malignantly transformed progenitors and that oval cells are partially or completely transformed progenitors (Zvibel I, Fiorino A, Brill S, and Reid L M. Phenotypic characterization of rat hepatoma cell lines and lineage-specific regulation of gene expression by differentiation agents. Differentiation 63:215-223, 1999).
Attempts have been made in the past to obtain the hepatic progenitor cell population, suggested to be the most versatile population for cell and gene therapy of the liver. U.S. Pat. Nos. 5,576,207 and 5,789,246 (Reid et al.) utilize cell surface markers and side scatter flow cytometry to provide a defined subpopulation in the liver. Subpopulations of rat hepatic cells have been isolated by removal of lineage-committed cells followed by selection for immature hepatic precursors which were detected as being agranular cells bearing OC.3-positive (oval cell antigenic marker), AFP-positive, albumin-positive, and CK19-negative (cytokeratin 19) cell markers. The foregoing rat liver subpopulations demonstrate particular characteristics important in isolation and identification of enriched hepatic progenitors from rodent liver.
Thus, there exists a need to develop methods of isolating human hepatic progenitors that may be used to treat patients with liver disease or dysfunction. The present invention satisfies this need and provides methods of treatment as well.